Downhole oilfield tools may be called upon to operate reliably and safely in a hostile environment. The downhole oilfield tools may operate under high pressure associated with a passive hydraulic pressure created by a several thousand foot column of drilling fluid in the wellbore. Temperature extremes can be encountered in different wellbores in different regions around the world. Sometimes the downhole oilfield tools may be called upon to operate in the presence of caustic chemicals that may have been introduced into the wellbore to encourage or stimulate production of hydrocarbons in well completion operations. The downhole oilfield tools may be installed onto a work string for deployment into the wellbore by marginally skilled and sometimes fatigued workers. Furthermore, the working environment at the surface of a wellbore may be dirty, cluttered, and unsuited to delicate, precise, and clean final assembly of precision and/or finicky downhole oilfield tools.
Some wellbores are cased by placing a string of casing pipe extending from the surface to a location near the bottom of the wellbore. A perforation gun is a type of downhole explosive tool that is directed to cutting orifices in the casing and further to cut some distance into the formation surrounding the wellbore to form channels by the use of an explosive charge. The hydrocarbons and/or other fluids trapped in the formation flow into the channels introduced into the formation by firing the perforation gun, into the casing through the orifices cut in the casing, and up the casing to the surface for recovery. In some circumstances multiple perforation gun sub assemblies may be connected to each other and fired in unison.
Because of the danger associated with the powerful explosive charges contained in a fully assembled, armed explosive tool, great care must be taken to assure safety in operation and transportation of fully assembled explosive tools. A fully assembled explosive tool may be vulnerable to several accidental firing scenarios. For example, an electrically initiated explosive tool may be subject to accidental firing in response to electrostatic shocks, such as those associated with lightning or build up of electrostatic charges resulting from friction between moving objects, or Radio Frequency energy in the surrounding environment. Some explosive tools may be subject to accidental firing in response to excessive heat, such as may be experienced in a fire, for example a fire caused by a vehicle accident.